Most electric devices such as notebook, desktop computer and PDA need a regulated voltage to power function blocks. DC-DC converters with COT control characterized with fast transient response and simple structure are widely used in the above fields. Typically, DC-DC converters with COT control need slope compensation to stabilize the output voltage.
A slope compensation signal has a fixed slope in prior art. It is reset to zero when a main power switch in the converter turns on. Then it increases with a fixed slope until the slope compensation signal is reset again when the output voltage of the converter drops to a sum of the slope compensation signal and a reference voltage. This process repeats during the operation of the converter.
However, if load step occurs (e.g. the load steps to heavy load from light load, or the load steps to light load from heavy load), the output voltage decreases rapidly and falls below the reference voltage in a short time period. FIG. 1 schematically shows time waveforms of the inductor current IL, the switching control signal PWM, the output voltage VO, the slope compensation signal Vsl, the reference voltage VREF and the output current IO in a typical DC-DC converter with COT control when the load suddenly steps to heavy load from light load, wherein the x axis represents time. As shown in FIG. 1, the load suddenly steps to heavy load from light load at time point t1. Then the slope compensation signal VSl is reset to zero several times and a plurality of switching control signals PWM are generated in short time period (from time point t1 to time point t2). This plurality of switching control signals PWM causes the inductor current to rise rapidly. Then the inductor would store much more power than needed after the new steady state is reached. The redundant power would charge an output capacitor, which pumps the output voltage VO, and causes an overshoot issue. In some worst situations, voltage ring back may occur.